Flux used for soldering is applied to the solder through a flux applying step when soldering. The flux chemically removes any metal oxides on a surface of the solder and a surface of the metal to be soldered, and forms intermetallic compounds between the solder and the metal to be soldered so that they can be strongly connected. Moreover, the flux has such effects that it prevents the metal surface from being again oxidized during a period of heating time in a soldering step; it reduces surface tension of the solder to improve wettability thereof.
Further, flux coating preform solder in which the flux is applied to a surface of the solder formed so as to be a long, narrow strip of solder has been proposed. Since in the flux coating preform solder, the flux has been previously coated, it is possible to omit a flux applying step in the soldering time. Since the flux coating preform solder is a long, narrow strip of solder, it can be processed so as to be a suitable form such as a pellet, a washer and a disk, thereby allowing it to be applied to a various kinds of mounting technologies.
In such flux coating preform solder, the flux needs to be coated thin and almost uniformly on a surface of solder. This is because if the flux is thick or non-uniformly applied on the surface of the solder, this becomes any cause of failure in the soldering. Patent Document 1 discloses a flux applying apparatus in which a solder line is passed through a tank containing molten flux and rollers pull the solder line through a die so that the die adjusts an amount of flux applied to the solder line on an assumption that the flux is applied to the surface of the solder line just before the soldering.